The present invention relates to an image processing device and method capable of enhancing the contrast of an image taken by an image sensor.
Typically, an image sensor having a specially wide dynamic range is needed to be used in order to take an image of a subject including a very light (highlight) portion and dark (shadow) neighboring portions, for example, when detecting a white divisional line indicated on a road in a view ahead of a vehicle running into or out of a tunnel or in the case of monitoring the conditions of a work being manufactured by welding or laser, plasma or flame machining.
Japanese Laid-open Patent Publication No. 2000-329616 discloses an image sensor having a logarithmic output characteristic to widen its dynamic range, which comprises a matrix of light sensor circuits each of which represents a unit pixel and works in such a way that sensor current corresponding to a quantity of incident light is produced and converted by a photodiode (photoelectric converting element) into an electric voltage having a logarithmic characteristic in a weak inverse state by using the subthreshold region property of the transistor and, then, a sensor signal corresponding to the voltage signal is produced and output.
The image sensor possessing a wide dynamic range of its logarithmic output characteristic can take an image of any subject with a wide range of brightness from a very light portion to a dark portion but cannot assure the sufficient contrast of the image because the brightness is logarithmically compressed.
For example, when an image taken by the above-described image sensor is displayed on a monitor screen and visually estimated, the logical processing of the image by a computer is needed in order to increase the image contrast but such processing takes much time, making it impossible to do real-time analysis of the image.